As described in copending application Ser. No 220,663, metal structures having UV sensitive indicating coatings can be made by plasma spraying a mixture of a UV sensitive metal oxide phosphor, for example, cerium magnesium aluminate doped with +3 terbium and a metal powder, for example, Metco 450Ni powder. The resulting coated metal structure can be used in a variety of applications subject to a high degree of surface erosion. As taught in copending application Ser. No. 220,663, the phosphor containing coating can serve as a UV sensitive indicating layer.
Experience has shown that in practicing the method of Ser. No. 220,663, commercially available UV sensitive phosphors often result in indicating coatings having substantially reduced fluorescence, as compared to the degree of fluorescence in the original unsprayed mixture of ingredients. Various explanations have been proposed as to the possible reason for the significant drop in fluorescence of the applied coating as compared to the original mixture prior to plasma spraying. One possible explanation is that the phosphor particles and metal particles in the initial plasma or flame sprayed mixture do not melt before they strike the metal substrate. This explanation is supported by D. A. Gerdeman et al, Arc Plasma Technology in Materials Science, Springer-Verlag, Wien, NY, 1972. This lack of proper melting can cause particles to bounce off the surface of the substrate, or to be blown out of the plasma stream and therefore not strike the substrate.
The present invention is based on the discovery that by plasma or flame spraying a mixture of a UV sensitive metal oxide phosphor and a plasma or flame sprayable metal, metal carbide, or metal oxide, where such ingredients are in a critical particle size range, as described hereinafter, the resulting ceramic or metal indicating coating has a substantially enhanced degree of fluorescence under UV light, as compared to plasma or flame sprayed coatings utilizing a mixture of such ingredients in a particle size range outside the critical range.